1. Field of the Invention
This invention relates to a microwave oscillator of the type in which a dielectric resonator is disposed inside a conductive housing.
2. Description of the Prior Art
Hitherto, the microwave oscillator composed of a dielectric resonator is widely used as the oscillator for generating frequencies of the microwave band. For reference, an example of the conventional microwave oscillator is shown in FIG. 4 in vertical sectional view. In FIG. 4, a microwave oscillator 1 is configured so that on the bottom of a metal frame 2 whose one face is opened a circuit substrate 3 is arranged, on this circuit substrate 3 a dielectric resonator 4 is arranged, and an electric circuit not shown is also provided which comprises active elements such as transistors and passive elements, such as resistors and condensers. To the open face of the metal frame 2 a metal cover 5 is attached to close the same. On the metal cover 5 a conductive adjustable screw 6 is mounted which is positioned coaxially with respect to the dielectric resonator 4 and adjustable so as to approach and separate from the dielectric resonator 4. By the metal frame 2 and metal cover 5 a conductive housing 7 is made up.
In the foregoing configuration, as the adjustable screw 6 is so adjusted as to approach or separate from the dielectric resonator 4, the resonance frequency of the dielectric resonator 4 is adjusted, so that the adjusted resonance frequency is selected and amplified by the electric circuit provided on the circuit substrate 3 and the selected oscillation frequency is output from the microwave oscillator 1. By means of the housing 7 undesirable emission of oscillation energy from the dielectric resonator 4 is prevented.
According to such a configuration, the dielectric resonator 4 produces an electric field E distributed about its axis as shown in FIG. 5 and a magnetic flux .PHI. passing in the axial direction thereof as shown in FIG. 6. Thus, as the adjustable screw 6 is adjusted and moved, the amount of magnetic flux .PHI. generated by the dielectric resonator 4 and interrupted by the adjustable screw 6 varies, as shown in FIG. 7 and the resonance frequency of the dielectric resonator 4 changes. If the adjustable screw 6 is so adjusted as to approach the dielectric resonator 4, for example, the resonance frequency of the dielectric resonator 4 becomes high.
In addition, the conductive housing 7 made up of the metal frame 2 and metal cover 5 forms a cavity resonator inside which an electric field E and magnetic field H are distributed as shown in FIG. 8. As the adjustable screw 6 is inserted into the electric field E, as shown in FIG. 9, a part of the electric field E is interrupted and the resonance frequency becomes low. Obviously, the resonance frequency of the cavity resonator formed by the housing 7 is determined by the shape and size of the housing 7, and the smaller the size, the higher the resonance frequency becomes.
Because the dielectric resonator 4 is stored in the housing 7, its Q suffers an influence of the housing 7, thereby becoming low. Therefore, in the prior art, the size of the sides of the housing 7 is selected larger than about two times the diameter of the dielectrc resonator 4 and the electric circuit provided on the circuit substrate 3 is designed so as to compensate for a decrease of Q; thus, it was impossible to miniaturize the microwave oscillator 1 to a great extent.
However, miniaturization of such equipments is strongly desired recently. In view of such a demand, if the microwave oscillator 1 of the conventional configuration were miniaturized further, a decrease of Q may be compensated for by the electric circuit provided on the circuit substrate 3; but, there arises the serious problem that the oscillation frequency of the microwave oscillator 1 becomes unstable.
The foregoing problem will now be described. As the housing 7 is made small in size, the resonance frequency of the housing 7 becomes high and sometimes exceeds the resonance frequency of the dielectric resonator 4. In such a case, as will become apparent from a return loss-frequency characteristic obtained when viewed the conventional microwave oscillator from its output terminal, as shown in FIG. 10, if the adjustable screw 6 is so adjusted as to approach the dielectric resonator 4, the resonance frequency of the housing 7 becomes low and contrarily, the resonance frequency of the dielectric resonator 4 becomes high; thus, there arises the state in which the resonance frequency of the housing 7 is identical substantially to that of the dielectric resonator 4. As a result, the respective resonance frequencies of the housing 7 and dielectric resonator 4 are amplified and output simultaneously by the electric circuit provided on the circuit substrate 3, whereby the two oscillation frequencies are output from the microwave oscillator 1. For reference, in FIG. 10, symbol A indicates the resonance frequency of the dielectric resonator 4 when the adjustable screw 6 is spaced sufficiently from the dielectric resonator 4 and B indicates the resonance frequency of the housing 7 in the above state, whereas symbol A' indicates the resonance frequency of the dielectric resonator 4 when the adjustable screw 6 is positioned close to the dielectric resonator 4 and B' indicates the resonance frequency of the housing 7 in this second state.